1. Field of the Invention
This invention relates to an electronic flash apparatus capable of illuminating a scene to be photographed, in general, and to such apparatus having a controlled and variable spectral light output, in particular.
2. Description of the Prior Art
The color balance of an image formed in a photosensitive material is dependent upon several factors. One factor is the color balance of the photosensitive material itself. The continuous manufacture of large quantities of photosensitive materials over extended periods of time, especially materials of the self-developing type with their large number of coating layers, requires fairly complex processes that are relatively difficult to control. One consequence of employing such complex processes is an occasional unwanted shift in the color balance from a nominal or desired color balance, a shift that normally produces an excessive level of one particular color in an image subsequently formed in such materials. A more detailed explanation of this problem is described in U.S. Pat. No. 4,329,411 to Land.
Image color balance is also affected by the color temperature of scene illumination. The color temperature may produce a concentration of light frequencies at the higher or lower energy portions of the visible light spectrum. For example, a scene having a relatively high color temperature will have scene light predominately composed of high frequency radiation at the blue end of the visible spectrum, whereas a scene having a relatively low color temperature will have scene light predominately composed of low frequency radiation at the red end of the visible spectrum.
Optical filters have been employed in the past for balancing the color of an image formed in photosensitive materials located within a photographic camera. In commonly assigned U.S. Pat. No. 4,736,215 to Hudspeth et al, for example, a film cassette is provided with indicia or machine readable information on an external surface thereof corresponding to one or more film variables of a film unit enclosed within the film cassette. A camera into which the film casette is insertable is provided with three optical filters for controlling photosensitive material color balance, each of which is selectively movable into the optical axis of a taking lens of the photographic camera under control of signals developed by reading means within the camera responsive to the film cassette indicia.
In commonly assigned U.S. Pat. No. 3,468,228 to Rogers, an automatic shutter mechanism for a photographic camera is disclosed which incorporates a selection of color balancing filters. The filters compensate for color balance shifts produced by scene color temperature. Optical filter systems that are effective in automatically controlling the color balance of an image formed in photosensitive materials have previously been incorporated in photographic apparatus. However, these optical filter systems significantly increase both the cost and size of the apparatus in which they are employed.
In U.S. Pat. No. 4,485,336 to Yoshiyama et al, for example, an electronic flash device is provided in which the color temperature of the flash of artificial light is controlled so that it can compensate for color imbalance in a photosensitive material or a color imbalance caused by scene color temperature. The electronic flash device includes three different xenon flash tubes with each such tube having a red, green or blue filter through which light from a xenon flash tube is transmitted. The particular flash tube and filter employed, and therefore the color of light emitted by the electronic flash device, is dependent upon an operator selected characteristic of the photosensitve material and/or the automatically sensed scene color temperature. While effective in compensating for photosensitive material and scene color temperature produced color imbalance, this electronic flash device is relatively complex and the multiple flash tubes require considerably more space than a flash arrangement where, for example, a single flash tube might be employed for such purposes.